Atomistic simulation of phonon heat transport across metallic vacuum nanogaps

The understanding and modeling of heat transport across nanometer and subnanometer gaps, where the distinction between thermal radiation and conduction becomes blurred, remains an open question. In this work, we present a three-dimensional atomistic simulation framework by combining the molecular dynamics (MD) and phonon nonequilibrium Green's function (NEGF) methods. The relaxed atomic configuration and interaction force constants of metallic vacuum nanogaps are generated from MD as inputs into harmonic phonon NEGF. Phonon tunneling across gold-gold and copper-copper nanogaps is quantified, and is shown to be a significant heat transport channel below a gap size of 1 nm. We demonstrate that lattice anharmonicity contributes to within 20%-30% of phonon tunneling depending on gap size, whereas electrostatic interactions turn out to have a weak effect for the small bias voltage typically used in experimental measurements. This work provides detailed information of the heat current spectrum and interprets the recent experimental determination of thermal conductance across gold-gold nanogaps. Our study contributes to deeper insight into heat transport in the extremely near-field regime, as well as hints for future experimental investigation.